Plant and microbial diversity are key to determine ecosystem functioning. Despite the well-known role of local-scale α diversity in affecting vegetation productivity, it still remains unclear about the effects of community heterogeneity (β diversity) of plants and soil microbes on above- and belowground productivity (AGB and BGB) across contrasting environments. Here, we conducted a dryness-gradient transect survey over 3000 km across grasslands on the Tibetan Plateau. We found that plant β diversity was more dominant than α diversity in stimulating AGB, while soil fungal β diversity was the key driver in enhancing BGB. However, these positive effects of plant and microbial β diversity on AGB and BGB were strongly weakened by increasing climatic dryness, mainly because higher soil available phosphorus caused by increasing dryness reduced both plant and soil fungal β diversities. Overall, these new findings highlight the critical role of above- and belowground β diversity in sustaining grassland productivity, raising our awareness to the ecological risks of large-scale biotic homogenization under future climate change.

The area of our transect survey spanned a latitude range from 30°14’24” to 33°12’9” N, in longitude from 79°48’43” to 96°44’24” E and in altitude from 3614m to 5016m across grasslands in the Tibetan Plateau of China. Specifically, we investigated 61 sampling sites during the peak growing season (July to August) of 2019 and 2020. These sites cover four grassland types, such as alpine meadow (AM), alpine typical steppe (ATS) and alpine desert steppe (ADS). The above vegetation communities were dominated by Kobresia pygmaea (AM), Stipa purpurea (ATS) and Stipa caucasica (ADS), respectively. Furthermore, the sampling sites have a large climate gradient, with mean annual precipitation (MAP) ranging from 854.8 mm to 92.7 mm and mean annual temperature (MAT) from -3.3 to 1.5℃. Soils are classified as Cambisol (World Reference Base for Soil Resources). The soil organic matter (SOM) ranged from 0.94-9.81%. Soil available nitrogen (AN) and available phosphorus (AP) ranged from 28.88-443.71 mg/kg and 0.95-7.11 mg/kg, respectively.

We obtained the latest 10-year mean annual precipitation (MAP) and mean annual temperature (MAT) data (2010–2020) from the China Meteorological Data Service Centre (CMDC; http://data.cma.cn), based on the located information of 61 sampling site. The evapotranspiration (ET) data (2010–2020) was downloaded from the Penman-Monteith-Leuning Version2 (https://developers.google.com/earthengine/datasets/catalog/CAS_IGSNRR_PML _V2) (Zhang et al. 2019). Then, we calculated the aridity index by mean annual ET divided by MAP to quantify climate dryness (UNESCO 1969; UNEP 1994; Huang et al. 2016). We also classified the aridity gradients into low aridity (aridity index: <1), middle aridity (aridity index: 1-1.2) and heavy aridity (aridity index: >1.2).